When I was still working I did a one week exchange with the Platform Manager at Gare du Nord. It was staggering how many people came off a train having boarded it in London for Brussels.
It was also amazing how many times a train would depart followed by - anything up to fifteen minutes later - a person or persons from the adjacent empty train asking why the train hadn't left yet. Polite questioning along the lines of, "didn't you notice that you were the only person in the coach"? or "didn't you notice everyone else boarding the other train"? usually elicited a blank look.